Ion Sensitive Field Effect Transistors (ISFETs) are often used to sense the concentration of a target ion or molecule in an electrolytic solution. In early ISFET designs, the fluid being sensed was often in intimate contact with the gate dielectric, or separated from the gate dielectric by an ion-sensitive membrane. Ultimately, however, CMOS process flows were used to create relatively low-cost ISFET structures with improved manufacturability.
A typical CMOS ISFET includes a sense plate and a CMOS transistor, which includes a floating gate that is coupled to the sense plate. As is typical with a CMOS transistor, the gate is electrically isolated from the transistor channel through a gate dielectric. During fabrication of a typical CMOS ISFET, once the gate dielectric is formed, it is susceptible to damage, particularly plasma-induced damage from fabrication processes in which plasma phenomena are present. These processes include, for example, plasma etching processes, plasma deposition processes, and so on. Even after fabrication, the gate dielectric may be susceptible to damage from electrostatic discharge (ESD) events or other situations in which the breakdown voltage of the gate dielectric is exceeded.
Accordingly, there is a need for improved ISFET devices that are less susceptible to plasma-induced and/or ESD-induced gate dielectric damage and which are manufacturable using conventional CMOS semiconductor processes.